Lithium batteries, which are repeatedly useable by charging, have been attracting much attention with recent expectation for environment conservation and carbon reduction. The lithium batteries have many benefits such as having higher energy density, being more lightweight and having longer life span as compared with conventional secondary batteries including nickel hydrogen batteries and lead storage batteries. For these reasons, they are widely used as power sources for electric devices such as mobile phones and notebook computers and electric power tools, and moreover they are focused also as a power source for hybrid automobiles and electric automobiles.
However, automobiles having a lithium battery mounted therein present a significant problem in its safety. A technique for enhancing the safety of the lithium battery that has been reported is the use of maleimide-based polymers (Patent Document 1).